When you say 'adding QALYs' you mean interventions which generate QALYs right? Not taking two interventions which generate QALYs and trying to estimate the combined effect of doing both interventions together?

If the former, there's a very interesting paper giving an overview of many different methods of generating QALYs here : https://pubmed.ncbi.nlm.nih.gov/7604170/

It is quite outdated now, and some of the assumptions about what does and doesn't count are controversial, however I still think it is an excellent way of thinking about the variety of different ways in which you might generate QALYs.

The general answer to your first question is, I think, that the most robust methods of generating QALYs are exactly what you would expect - laws mandating seat belt usage, fire alarm installations, vaccinations etc

With respect to air pollution, you'll see that there are significantly varying estimates for cost-effectiveness depending on exactly the intervention used to control pollution. For example, 'Coal-fired power plants emission control through high stacks' is approximately as cost-effective as wearing a seatbelt whereas 'Acrylonitde emission control via best available technology' is one of the least cost-effective interventions studied. The references attached to each intervention will give more details on how these estimates were arrived at.

With regards to your second question, if you search for air pollution using the forum's search tool you'll see there have been some recent posts on the topic, including this post on a particular intervention to reduce air pollution, and this high-level overview of the topic by Founders Pledge. The latter seems to include a figure of 213 million DALYs, from IMHE.